The lighting of a modern aircraft cabin is nowadays subject to very great demands. For the coloured configuration of the cabin, attention should be paid in particular to exact colour locus reproduction and brightness compliance. Likewise, the often desired adjustability of the brightness in a range of from 0.1% to 100% places great requirements on the controlling of illumination devices. This applies in particular when carrying out a gamut colour locus correction, which is indispensable for colour locus adjustment or for manufacturing adjustment. To this end, the individual colours would need to be controllable with a brightness which is significantly less than 0.1%. The effect of this, in particular, is that lighting means without, or with reduced or insufficient gamut correction, operate at low brightnesses and a satisfactory gamut correction is only achieved with higher brightness.
It is expected that the requirements relating to the colour locus at low brightness are constantly increasing. For many types of aircraft, for example, a maximum colour difference of less than 6 threshold value units of a MacAdam ellipse is required as far as interior illumination is concerned, the colour difference also being necessary in a brightness range of from 0.1 to 1%.
The brightness control of cabin illumination is conventionally carried out by means of so-called pulse-width modulators. The minimum duty cycle of a pulse-width modulator is generally determined by the switching behaviour of the current source, and is conventionally established at about 1 μs. Contemporary pulse-width modulators are driven by means of a microcontroller, the shortest clock period of which may for example be 50 ns. For a minimum duty cycle of 1 μs of the pulse-width modulator, this gives merely 20 clock periods for the microcontroller. If the pulse-width modulator is then increased by just one clock period, the duty cycle changes by 5%. Consequently, for low obscuration levels (dimming levels) the colour locus cannot be adjusted exactly, or the brightness can only be adjusted in stages.
A possible improvement of the brightness control could consist in providing a further current source in order to avoid the restriction of the minimum duty cycle by the switching behaviour of the current source. Although the minimum change in the brightness control could thereby be about halved, the installation of a further current source would however be expensive.